Film of aromatic polyethersulfone, process for the production thereof, and solution composition for the production thereof

ABSTRACT

An optically isotropic film of an aromatic polyethersulfone, which is useful as a substrate for a liquid crystal display device, and an optically anisotropic, uniaxially oriented film of an aromatic polyethersulfone, which is useful as a phase difference plate for a liquid crystal display device. The optically isotropic film is produced by casting an aromatic polyethersulfone solution composition comprising 15 to 40 parts by weight of a solvent which contains at least 60% by weight of 1,3-dioxolane and can dissolve an aromatic polyethersulfone and 10 parts by weight of the aromatic polyethersulfone, on a substrate, and heating the cast solution composition containing a solvent to evaporate the solvent off.

This is a division of application Ser. No. 08/415,136, filed Mar. 30,1995 now U.S. Pat. No. 5,645,766.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to an aromatic polyethersulfone filmuseful in the optical field of a display device or in the field ofelectric and electronic equipment and machinery, and a process for theproduction thereof. More specifically, it relates to a solution castingmethod for the production of an aromatic polyethersulfone film havingexcellent surface properties, transparency and optical homogeneity andhaving a reduced residual solvent, from a solution composition (dope)containing 1,3-dioxolane which is a lo halogen-free solvent free fromcausing environmental pollution, and a film produced by said method.

In recent years, a liquid crystal display device attracts attention andis being put to practical use due to its small consumption of electricpower and excellent image qualities. In the liquid crystal displaydevice, a polymer film is used as a polarization plate, a protectionlayer, a phase difference plate (a retardation plate) and an electrodesubstrate. Of these, a polymer electrode substrate, i.e., a plasticsubstrate is used as a substitute for a conventional glass substrate,for decreasing the weight and thickness of the liquid crystal displaydevice, and it is therefore required to have very high optical isotropyand uniformity for accurately transmitting polarized light beingtransmitted therethrough. The polymer electrode substance is furtherrequired to have heat resistance sufficient for withstanding heatapplied when a film of a transparent electrode is formed or anorientation film is formed. For this reason, an unstretchedpolycarbonate film or polyarylate film is used, and a film of anaromatic polyethersulfone is also considered promising since itremarkably shows excellent properties in view of heat resistanceTAKA-HASHT Junji, "Recent Tendency toward Development of ElectrodeSubstrate Film", Polymer Society, Excerpts of Study of PolymerElectronics, p. 20 (Nov. 11, 1993; in Sophia University)!. However, thedefect of an aromatic polyethersulfone film is that, since it is highlypolarizable due to an aromatic group contained in its molecule, it showsoptical anisotropy even when its molecules are slightly oriented. Underthe circumstances, it is a very important theme to develop a techniquewhich enables the production of an aromatic polyethersulfone film havingexcellent optical isotropy with preventing the molecular orientation asmuch as possible.

On the other hand, a phase difference film is used for improving thevisibility of an image in an STN liquid crystal display device or a TNliquid crystal display device, and it has a role of convertingelliptically polarized light transmitted through a liquid crystal layerto linear polarized light. A uniaxially oriented polycarbonate film orpolyvinyl alcohol film is mainly used as a material for a phasedifference film of STN liquid crystal display device. To cope withdemands for improving the image visibility of a fast-responding liquidcrystal display, there is required a phase difference film having awavelength dispersibility of a phase difference (retardation) agreeingwith a high-speed liquid crystal layer, and an aromatic polyethersulfonefilm is considered highly promising MURAYAMA Akio et al, "OpticalDesigning of Simple Matrix STN-LCD", Polymer Society, 2nd PolymerMaterial Forum. p. 267 (Dec. 1, 2, 1993 in National Education Hall,Tokyo)!. As described above, aromatic polyethersulfone is highlypolarizable since its molecule contains an aromatic group, and istherefore easily imparted with a refractive index anisotropy bymolecular orientation by subjecting the film to uniaxial stretching tocause molecular orientation. Therefore, the advantage of the aromaticpolyethersulfone film is that the phase difference required of a phasedifference film can be obtained ban small stretching, while it isdifficult to obtain an optically uniform oriented film. For obtainingsuch an optically uniform oriented film, it is required to use anunstretched film (base film) having highly optically isotropy, and it istherefore desired to develop a competent film-forming technique.

For improving the visibility of an STN liquid crystal display device,refractive index anisotropy in an in-plane area of a film is required asdescribed above. However, for improving the visibility withoutdependency on a visual angle, it is required to increase the refractiveindex (n_(z)) in a perpendicular direction relative to the film surfaceFUJIMURA Yasuo et al, "STN-LCD Phase Difference Film", "Denshi Zairyo(Electronic Material)", February 1991, p. 37!. In films having anaromatic skeleton structure including a polycarbonate film, n_(z) isvery small due to so-called plane orientation, as compared with theminimum refractive index in the in-plane area, i.e., a refractive index(n_(f)) in a fast axis direction. In view of these points, it is desiredto bring an n_(z) /n_(f) value close to 1 when the film is formed. Inother words, it is desired to produce a film having three-dimensionaloptical isotropy including the refractive index (n_(z)) in aperpendicular direction with regard to the film plane.

Generally, an aromatic polyethersulfone film is formed by a meltextrusion method, particularly a T-die method (with flat film die). TheT-die method is widely used as a method for producing plastic films.However, since a melt having a high viscosity is extruded in thismethod, a polymer chain is liable to be oriented and a stress strain isliable to remain in the film, so that it is difficult to obtain opticalisotropy or uniformity. For decreasing the melt viscosity, it isrequired to decrease the molecular weight of a plastic or increase thetemperature for forming the film. However, when the molecular weight isdecreased, the film shows decreased mechanical properties, and when thefilm-forming temperature is increased, the film is liable to undergothermal deterioration or coloring. Further, since an extrudate from aT-die is directly and rapidly cooled, the film is liable to have streakscaused by the T-die, so-called T-die lines, and it is thereforedifficult to produce a film having high surface properties TAKAHASHIJunji, "Recent Tendency toward Development of Electrode Substrate Film",Polymer Society of Japan, Excerpts of Study of Polymer Electronics, p.20 (Nov. 11, 1993; in Sophia University)!. The surface properties andoptical uniformity required of a film used in a liquid crystal displaydevice are considerably severe. For example, a film for a plasticsubstrate is required to have a surface thickness nonuniformity of up to±5 μm, a phase difference (retardation) of 10 nm or less and an opticalorientation angle of up to ±10°. A base film for a phase difference filmis required to have a surface thickness nonuniformity of 2 μm or lessand a phase difference of 30 nm or less. In present practice, it isdifficult to accomplish these severe requirements by a melt extrusionmethod.

For avoiding the above problems, it is expected that the film is formedby a solution casting method. For producing a thick film having athickness of about 100 μm such as a film for a liquid crystal displaydevice, by a solution casting method, a solution (dope) having a highconcentration is required. However, there is limitation on the selectionof the solvent which can dissolve an aromatic polyethersulfone in a highconcentration, can be relatively easily dried and can be used forforming a film (Henry Lee, Donald Stoffey and Kris Neville, New LinearPolymers, McGraw-Hill, p. 107). For example, the solvent is selectedfrom polar aprotic solvents (e.g., dimethylacetamide, dimethylformamldeand N-methylpyrrolidone) , acetophenone, chlorobenzene andcyclohexanone. Since, however, these solvents have a high boiling point,the amount of a residual solvent in the formed film cannot be easilydecreased, and these solvents are not suitable as the film-formationsolvent for practical use. Cyclic ethers such as 1,4-dioxane andtetrahydrofuran can dissolve some kind of aromatic polyethersulfone.However, 1,4-dioxane is not that which has a low boiling point, andfurther, it is difficult to use 1,4-dioxane from a practical point ofview since it is carcinogenic. Tetrahydrofuran is free of the problemsof a boiling point and carcinogenic nature. However, it does not give ahigh solution viscosity and hence, it is difficult to form a uniformfilm. Halogen-containing solvents such as dichloromethane and chloroformmay be selected as a good solvent, while these solvents involve theproblem of environmental pollution, and are under suspicion as one ofcarcinogens. These solvents therefore tend to be prohibited from use.Further, since these solvents work as a corrosive compound during along-term use even if the amount of them remaining in the film is verysmall, and the use thereof is therefore limited when they are used for adisplay device and an electric or electronic apparatus in which fineelements are used. In view of these points, it is expected to develop atechnique for the production of a film from a halogen-free solvent.

Processes for the production of polysulfone films by a casting methodare disclosed, for example, in the following Japanese Laid-open PatentPublications.

JP-A-60-137617 discloses a process for the production of an aromaticpolyethersulfone film, which comprises dissolving an aromaticpolyethersulfone in a mixed solvent containing a mixture of at least twokinds of halogen-containing solvents and an aliphatic alcohol and/or analiphatic ester, casting the resultant solution on a substrate, and thenremoving the solvents.

JP-A-60-137618 discloses a process for the production of apolyethersulfone film, which comprises casting a polyethersulfonesolution on a substrate, drying the solution until it forms aself-supporting film, peeling off the film from the substrate, andfinally drying and heat-treating the film in the temperature range of±30° C. of the glass transition temperature of the polyethersulfonewithout applying a tension of 500 g/cm² or higher. It is disclosed thatthe solvent for the polyethersulfone is selected from amide-containingsolvents such as N,N-dimethylformamide and halogen-containing solventssuch as dichloromethane and chloroform.

JP-A-60-137619 discloses a process for the production of an aromaticpolyethersulfone film, which comprises casting an aromaticpolyethersulfone solution having a rotational viscosity of 10² to 2×10⁵centipoises at 25° C. on a substrate with a blade applicator andremoving a solvent. It is disclosed that the solvent for thepolyethersulfone is selected from amide-containing solvents such asN,N-dimethylformamide, cyclic, nitrogen-containing compounds such asN-methyl-2-pyrrolidone, chlorine-containing compounds such asdichloromethane and phenols such as p-chlorophenol.

JP-A-60-138514 discloses a process for the production of a liquidcrystal cover film having a retardation value of 20 nm or less and acenterline average roughness, on the surface, of 0.2 μm or less, whichcomprises casting an aromatic polyethersulfone solution having arotational viscosity of 10² to 2×10⁶ centipoises at 25° C. on asubstrate, drying the solution until it forms a self-supporting film,peeling off the film from the substrate and finally drying andheat-treating the film at a temperature equivalent to, or higher than,the boiling point of a solvent until the amount of residual solvent isreduced to 5% by weight. It Is disclosed that the solvent is selectedfrom those disclosed in the above JP-A-60-137617.

JP-A-60-228113 discloses a process for the production of an aromaticpolyethersulfone film, which comprises casting an aromaticpolyethersulfone solution having a rotational viscosity of 5×10³ to 10⁶centipoises at 25° C. on a substrate through a slit having an opening of0.1 to 2 mm at an average casting rate of 0.1 to 10 m/minute andremoving a solvent. It is disclosed that the solvent is selected fromthose disclosed in the above JP-A-60-137619.

JP-A-61-204234 discloses a process for the production of an opticallyisotropic polysulfone film, which comprises casting a solution of apolysulfone in methylene chloride, drying the solution until theresidual volatile content is 20% by weight or less, and peeling off aformed film from the substrate.

JP-A-3-58825 discloses a process for the production of a stretched film,which comprises drying and stretching an unstretched film or sheetformed from a polyethersulfone solution by a casting method, in a statewhere the amount of a residual solvent is in the range of 2 to 20% byweight. It is disclosed that the solvent is selected fromdimethylformamide, dimethylacetamide and 1,1,2-trichloroethylene.

JP-A-6-79736 discloses a process for the production of apolysulfone-based resin film, which comprises casting apolysulfone-based resin solution having a water content of 0.2% byweight or less and drying the solution. It is disclosed that the solventfor dissolving the polysulfone-based resin film is selected frommethylene chloride, 1,2-dichloroethane and chlorobenzene.

JP-A-5-239229 discloses a process for the production of a polysulfonefilm or sheet, which comprises peeling off a polysulfone film or sheetformed on a metal substrate by a solution casting method, from the metalsubstrate when the amount of a residual solvent has reached 20% byweight or less. It is disclosed that the solvent is selected fromhalogenated hydrocarbons such as methylene chloride; hydrocarbons suchas hexane and benzene; esters such as ethyl acetate; ketones such asmethyl ethyl ketone; alcohols such as cresol and isopropanol; sulfoxidessuch as DMSO; N-methylpyrrolidone and water.

JP-A-7-24858 discloses a process for the production of a film of apolysulfone resin, which comprises forming a film from a solution of apolysulfone resin in acetophenone or N-methylpyrrolidone by a solutioncasting method.

It is an object of the present invention to provide an opticallyisotropic film of an aromatic polyethersulfone which is excellent insurface properties, transparency and optical uniformity and which has areduced residual solvent.

It is another object of the present invention to provide an opticallyanisotropic, uniaxially oriented film of an aromatic polyethersulfone.

It is further another object of the present invention to provide asubstrate and a phase difference plate (a retardation plate) for aliquid crystal display, as products from the above film of the presentinvention.

It is yet another object of the present invention to provide a processfor the production of an optically isotropic film of the presentinvention by a solution casting film-forming method without using anyhalogen-containing solvent which may cause environmental pollution orcorrosion.

It is still further another object of the present invention to provide asolution composition of an aromatic polyethersulfone useful as a dopefor the production of a film.

Other objects and advantages of the present invention will be apparentfrom the following description.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows the relationship of the glass transition temperature (Tg)of the aromatic polyethersulfone to the residual solvent content of thearomatic polysulfone.

According to the present invention, the above objects and advantages ofthe present invention are achieved, first, by an optically isotropicfilm of an aromatic polyethersulfone, which

(A) comprises an aromatic polyethersulfone,

(B) has a transmittance to visible light at a wavelength of 550 nm inthe range of from 86 to 91%,

(C) has a haze value in the range of from 0.1 to 1%,

(D) shows a refractive index difference (Δn) of 0.0010 or less, which isa difference between a refractive index (n_(s)) in the slow axisdirection in an in-plane area of the film and a refractive index (n_(f))in the fast axis direction in the in-plane area,

(E) has a refractive index ratio (n_(z) /n_(f)) of 0.9997 to 1.0000,which is a ratio (n_(z) /n_(f)) of a refractive index (n_(z)) in theperpendicular direction with regard to a film surface to the refractiveindex (n_(f)),

(F) has a thickness in the range of from 30 to 300 μm, and

(G) has a thickness nonuniformity in the range of from 0.1 to 1% basedon an average film thickness.

The above optically isotropic film of the present invention can beprovided for the first time on the basis of the fact which the presentinventors have found for the first time, i.e., the fact that (1)1,3-dioxolane can dissolve an aromatic polyethersulfone in a highconcentration and that (2) the resultant solution having a specificviscosity can give an optically isotropic flat film which is transparentwithout becoming opaque.

That is, according to the present invention, the above opticallyisotropic film of the present invention can be produced by a process ofthe present invention, which comprises

(1) casting an aromatic polyethersulfone solution composition containing15 to 40 parts by weight of a solvent which contains at least 60% byweight of 1,3-dioxolane and can dissolve an aromatic polyethersulfone,and 10 parts by weight of an aromatic polyethersulfone, on a substrate,and

(2) heating a cast film containing the solvent, to evaporate off thesolvent.

The aromatic polyethersulfone used in the above step (1) of the processof the present invention generally refers to aromatic polyethersulfonesin which aromatic groups are bonded to a skeleton through a sulfonegroup and an ether group. Such aromatic polyethersulfone is, forexample, an aromatic polyethersulfone having at least one kind ofrecurring unit(s) selected from the group consisting of the followinggeneral formulae (1) to (3),

    (-Ar.sup.1 -SO.sub.2 -Ar.sup.2 -O-)                        (1)

    (-Ar.sup.3 -X-Ar.sup.4 -O-Ar5-SO.sub.2 -Ar.sup.6 -O-)      (2)

    (-Ar.sup.7 -SO.sub.2 -Ar.sup.8 -O-Ar.sup.9 -O-)            (3)

wherein, in the formula (1), each of Ar¹ and Ar² is independently anaromatic hydrocarbon group having 6 to 12 carbon atoms, in the formula(2). each of Ar³ to Ar⁶ is independently an aromatic hydrocarbon grouphaving 6 to 12 carbon atoms and X is a divalent hydrocarbon group having1 to 15 carbon atoms, and in the formula (3) , each of Ar⁷ to Ar⁹ isindependently an aromatic hydrocarbon group having 6 to 12 carbon atoms.

In the above formula (1), each of Ar¹ and Ar² is preferably an arylenegroup having 6 to 12 carbon atoms, more preferably an arylene grouphaving 6 to 10 carbon atoms. Specific examples thereof includem-phenylene, p-phenylene, dimethyl-p-phenylene, tetramethyl-p-phenylene,naphthalene and biphenylene groups. A polyethersulfone of the formula(1) in which both Ar¹ and Ar² are p-phenylene groups is advantageous inview of production, and particularly preferably used.

In the formula (2), each of Ar³ to Ar⁶ is preferably an arylene grouphaving 6 to 12 carbon atoms, more preferably an arylene group having 6to 10 carbon atoms. Specific examples thereof include m-phenylene,p-phenylene, dimethyl-p-phenylene, tetramethyl-p-phenylene, naphthyleneand biphenylene groups. Each of Ar³ to Ar6 is particularly preferably ap-phenylene group. X is a divalent hydrocarbon group having 1 to 15carbon atoms, and is selected from an aliphatic hydrocarbon group having1 to 15 carbon atoms, an alicyclic hydrocarbon group and an aralkylenegroup. X is preferably a divalent aliphatic hydrocarbon group having 1to 10 carbon atoms, more preferably 3 to 9 carbon atoms, an alicyclichydrocarbon group or an aralkylene group. Specific examples of X includealiphatic hydrocarbon groups such as methylene, 1,1-ethylene,2,2-propylene, 2,2-butylene and 4-methyl-2,2-pentylene groups, alicyclichydrocarbon groups such as 1,1-cyclohexylene and3,3,5-trimethyl-1,1-cyclohexylene groups, and aralkylene groups such as1-phenyl-1,1-ethylene and diphenyl methylene groups. Of these, a2,2-propylene group is more preferred. In the formula (2), particularlypreferably, each of Ar³ to Ar⁶ is a p-phenylene group and X is a2,2-propylene group.

In the formula (3), each of Ar⁷ and Ar⁸ is preferably an arylene grouphaving 6 to 12 carbon atoms, more preferably an arylene group having 6to 10 carbon atoms. Specific examples thereof include an m-phenylene,p-phenylene, dimethyl-p-phenylene, tetramethyl-p-phenylene, naphthyleneand 4,4'-biphenylene groups. Of these, each of Ar⁷ and Ar⁸ is furtherpreferably a p-phenylene group. Ar⁹ is preferably an arylene grouphaving 6 to 12 carbon atoms, more preferably an arylene group having 6to 10 carbon atoms. Specific examples thereof include m-phenylene,p-phenylene, naphthylene and 4,4'-biphenylene groups. Of, these,p-phenylene and 4,4'-biphenylene groups are further preferred. In theformula (3), particularly preferably, each of Ar⁷, Ar⁸ and Ar⁹ is ap-phenylene group.

The aromatic polyethersulfone used in the present invention alsoincludes a composition or a copolymer comprising at least one kind ofrecurring unit(s) of the above formulae (1) to (3). For example, whenthe aromatic polyethersulfone is a copolymer, preferred is an aromaticpolyethersulfone having recurring units of the formulae (1) and (2) oran aromatic polyethersulfone having recurring units of the formulae (1)and (3). In this case, the proportions of recurring unit of the formula(1) and recurring unit of the formula (2), or the proportions ofrecurring unit of the formula (1) and recurring unit of the formula (3),i.e., the copolymerization composition ratio (1)/(2) or (1)/(3), can bedetermined in views of the solubility and heat resistance of an aromaticpolyethersulfone to be obtained and the physical properties of a formedfilm. Although the above copolymerization composition ratio is notspecially limited, preferred is an aromatic polyethersulfone containing0.1 to 99.9 mol %, preferably 1 to 99 mol %, of recurring unit of theformula (1).

Of the above-described aromatic polyethersulfones, aromaticpolyethersulfones of the following types are particularly preferred inview of availability, heat resistance and solubility. That is,particularly preferred are (i) a copolymer containing 70 to 90 mol % ofrecurring unit of the formula (1) in which each of Ar¹ and Ar² is ap-phenylene group and 30 to 10 mol % of recurring unit of the formula(3) in which each of Ar⁷ to Ar⁹ is a p-phenylene group, (ii) a polymerhaving recurring unit of the formula (2) in which each of Ar³ to Ar⁶ isa p-phenylene group and X is an iso-propylidene group (CH₃)₂ C<!, and(iii) a copolymer containing at least 70 mol % of recurring unit of theformula (2) in which each of Ar³ to Ar⁶ is a p-phenylene group and X isan isopropylidene group and 30 mol % or less of recurring unit of theformula (1) in which each of Ar¹ and Ar² is a p-phenylene group.

The above aromatic polyethersulfone can be produced by a knownpolymerization method. For example, it can be obtained by polycondensinga monomer having terminals of hydroxyl and halogen groups in a polaraprotic solvent in the presence of an alkali metal carbonate.

The molecular weight of the aromatic polyethersulfone used in thepresent invention is, when expressed by ηsp/c, 0.1 to 10 dl/g,preferably 0.3 to 5.0 dl/g. The viscosity values are those measured in a0.5 g/dl 1,3-dioxolane solution at 30° C. When this viscosity is smallerthan 0.1 dl/g, no durable film can be obtained. When it exceeds 10 dl/g,undesirably, not only it is difficult to obtain the above polymer, butalso the solubility of the aromatic polyethersulfone decreases.

The solvent used in the step (1) of the process of the present inventioncontains 1,3-dioxolane as a main component. This solvent has highsolubility and a relatively low boiling point, and it can be suitablyused as a solvent which gives a solution having a high concentration.Further, since it is a halogen-free solvent, it is almost free fromcausing environmental pollution and cancers which a halogen-containingsolvent may cause, and moreover, is free of the following problems. Thatis, a halogen-containing solvent may generate a corrosive gas typifiedby hydrogen chloride by reacting, for example, with water in air, andmay corrode a casting apparatus, particularly specular-finished metalcast drum and cast belt surfaces which can be said to be the heart ofthe apparatus. Further, when a film is formed from a solutioncomposition containing a halogen-containing solvent, a trace amount ofhalogen ion remaining in the film may deteriorate the transparentelectrode and TFT (thin film transistor) of a liquid crystal displaydevice. 1,3-Dioxolane used in the present invention is a halogen-freesolvent, and therefore is free of these possibilities.

The solvent used in the present invention is a solvent containing atleast 60% by weight, preferably at least 70% by weight, of1,3-dioxolane, and a solvent containing 1,3-dioxolane alone, i.e., asolvent containing 100% by weight of 1,3-dioxolane, is preferred. Othersolvent which may be contained in the solvent used in the presentinvention is not specially limited, and it may be selected in the lightof effects. The "effects" refer to effects on the improvement in thesurface properties (leveling effect) of a film formed by a solutioncasting method, and effects on the evaporation rate, viscosityadjustment of the system and inhibition of the crystallization, when thesolvents are mixed in such a range that the solubility and the stabilityare not impaired. The kind and amount of a solvent to be mixed can bedetermined depending upon the degrees of these effects. Further, atleast two solvents may be mixed with 1,3-dioxolane. The solvent to bemixed with 1,3-dioxolane preferably includes cyclic ethers such as1,4-dioxane and tetrahydrofuran, hydrocarbon solvents such as tolueneand xylene, ketone solvents such as acetone, methyl ethyl ketone andcyclohexanone, ester solvents such as ethyl acetate and butyl acetate,and ether solvents such as ethylene glycol dimethyl ether andmethoxyethyl acetate.

In the dope containing the aromatic polyethersulfone used in the presentinvention, the amount of the solvent per 10 parts by weight of thearomatic polyethersulfone is 15 to 40 parts by weight, preferably 20 to35 parts by weight. When the amount of the solvent exceeds the aboveupper limit, the effective concentration of the aromaticpolyethersulfone is undesirably low although the solution is stable.Moreover, when a film is formed from this solution composition by asolution casting method, undesirably, a rippling based on an externaldisturbance is liable to occur since the solution has a low viscosity,and it is difficult to obtain a film having surface flatness. When theamount of the solvent is less than the above lower limit, it isdifficult to obtain a stable dope. The above concentration is determinedmainly in view of the dope stability and the solution viscosity.

For easily peeling the film off from the substrate at the step (2) to bedescribed later and for promoting the precipitation of a crystallizablecyclic oligomer contained in the polymer in the step (2), it ispreferred to use water and/or an alcohol. The aromatic polyethersulfoneused in the present invention has high adhesion to a metal plategenerally used as a substrate. As a result, when the cast film is driedon the substrate and peeled off from the substrate to transfer it to aheat treatment step, it is difficult to peel off the film from thesubstrate. This problem naturally does not occur if the substrate isfrequently cleaned, while this frequent cleaning step is an additionaltroublesome step and hence, is undesirable. The addition of a smallamount of water and/or an alcohol has one significance that it canreduce the substrate-cleaning frequency. Another significance is that acrystal is precipitated. The aromatic polyethersulfone used in thepresent invention contains a small amount of a crystallizable cyclicoligomer such as the following compound. ##STR1## The above compoundgradually precipitates from the dope after the polymer is dissolved.Naturally, there is basically no problem generally industrially, sincethe dope is filtered halfway in the feeding of the dope from a dopestorage tank to a film-forming head before use. However, a crystalprecipitates in a site where a filtered dope is liable to reside so thatthe transparency of a film is often impaired. It is therefore importantto precipitate most of crystals as soon as possible after the solutionis prepared, and to manage to decrease the precipitation rate after thedope is filtered. For this purpose, it is preferred to use a linear orbranched aliphatic alcohol having 1 to 6 carbon atoms, preferably 1 to 4carbon atoms, such as methanol, ethanol, isopropanol or tertiarybutanol. Higher alcohols having more carbon atoms than the abovealcohols are not preferred since they have a high boiling point and areliable to remain after the film is formed. Further, it is not requiredto use a large amount of water and the alcohol. Generally, water and thealcohol are further added in an amount of 1 to 10% by weight, preferably2 to 8% by weight, based on the total amount of the solvent. When theamount of water and the alcohol exceeds the above upper limit,undesirably, a polymer precipitates. When it is less than the abovelower limit, there is no remarkable effect on the peeling of a film andthe precipitation of a crystal.

When the prepared dope contains insolubles or floating substances suchas impurities, or when the dope has a haze, it is desirable to removethem by treatment such as filtration. When this treatment is not carriedout, the formed film may have degraded optical properties. Further, theprepared dope per se may be degraded in storage stability.

In the process of the present invention, the aromatic polyethersulfoneis dissolved in the solvent containing 1,3-dioxolane as a main componentto prepare a solution composition (dope), and the dope is cast on asubstrate and heated to evaporate off the solvent, whereby a film isobtained. The industrial continuous film-forming process generallycomprises three steps such as a casting step, a pre-drying step and apost-drying step. The casting step is a step in which the dope is castflat and smooth, the pre-drying step is a step in which most of thesolvent is removed from the cast dope by evaporation, and thepost-drying step is a step in which the remaining solvent is removed.

In the casting step (1), there is used a method using a doctor blade, amethod using a reverse roll coater, or a method of extruding the dopethrough a die. It is industrially the most general practice tocontinuously extrude the dope through a die onto a belt-shaped ordrum-shaped substrate. Although not specially limited, the substrate isselected from a glass substrate, a stainless steel or ferrotype metalsubstrate and a plastic film of polyethylene terephthalate. Forindustrially obtaining a highly optically isotropic and excellentlyhomogeneous film which the present invention seeks to produce, aspecularly finished metal substrate is the most generally used.

The viscosity of a solution is generally an essential factor for forminga transparent and flat film from a dope. The solution viscosity differsdepending upon the concentration and molecular weight of the resin andthe kind of the solution, while the viscosity of the solutioncomposition used in the present invention is preferably 500 to 50,000cps, more preferably 700 to 30,000 cps. When this viscosity exceeds theabove upper limit, undesirably, the solution shows decreased flowabilityso that no flat film is obtained in some cases. When it is less than theabove lower limit, the flowability is so high that it is difficult touniformly extrude the solution through a T-die generally used forcasting, or that a surface nonuniformity occurs due to a rippling basedon an external disturbance. As a result, no uniform and flat film can beobtained.

Before proceeding with the drying step after the casting step, thedrying is suspended for a certain period of time to secure theflowability of the dope, whereby the film surface can be highlyflattened (leveling effect). In this case, a volatile solvent having alow boiling point, such as dichloromethane or chloroform, remarkablyevaporates even at room temperature. As a result, a rippling is inducedby the evaporation, and at the same time, the surface is dried to causean orange peel surface phenomenon. Since, however, 1,3-dioxolane used inthe present invention has a proper boiling point and proper volatility,such a phenomenon scarcely occurs, and it is preferred for producing afilm which is required to have high flatness.

In the pre-drying step, it is required to remove most of the solventfrom the dope cast on the substrate by evaporation as promptly aspossible. However, if the evaporation sharply occurs, the film suffersdeformation due to foaming, and it is necessary to set drying conditionscarefully. In the present invention, it is preferred and advantageous toinitiate the drying at a temperature in the range whose upper limit isequal to, or preferably 10° C. lower than, the boiling point of thesolvent which has the lowest boiling point among solvents used, andthereafter to increase the drying efficiency by increasing thetemperature thereafter. The upper limit of the temperature at a finalstage in this step is 120° C., preferably 100° C. In this step, when aresidual solvent is contained in a large amount, the amount is as muchas 25% by weight. Therefore, undesirably, a foaming lakes place when thetemperature is set at a temperature higher than the above upper limit.Further, air may be fed as required. In this case, the feed rate of airis generally up to 20 m/second, preferably up to 15 m/second. When thefeed rate of air exceeds the above upper limit, undesirably, flatsurface can not be obtained due to rippling caused by the air. The feedrate of air may be increased stepwise or continuously, and it is ratherpreferred to do so. At an initial state, no air may be fed for avoidingthe rippling caused by the air. The film is present on the substrate atthis stage, and is peeled off from the substrate at the end of thisstep. At this occasion, when the residual solvent amount is too large,the film is soft so that it is deformed. When the residual solventamount is too small, the adhesion of the film to the substrate is highso that the film has a stress strain upon peeling-off. Therefore, theresidual solvent amount is preferably 5 to 25% by weight, morepreferably 7 to 20% by weight.

In the post-drying step, the film which has been peeled off from thesubstrate is required to be further dried until the residual solventamount is 3% by weight or less, preferably 1% by weight or less, morepreferably 0.5% by weight or less. When the residual solvent amount islarge, the film undergoes deformation with time, or it undergoes adimensional change, so-called thermal shrinkage, then heated in apost-treatment step. For a liquid crystal display de-vice in particular,an optically uniform film is required and hence, it is required tocontrol the heat treatment temperature strictly. Generally, the heattreatment temperature is in the range of from (Tg-120° C.) to Tg,preferably from (Tg-100° C.) to (Tg-10° C.) in terms of the glasstransition temperature of the aromatic polyethersulfone used being Tg(°C.). When the heat treatment temperature is higher than the aboveupper limit, undesirably, the film undergoes heat deformation. When itis lower than the above lower limit, undesirably, the drying is tooslow. Generally, occurrence of the heat deformation becomes less with adecrease in the residual solvent amount. It is therefore preferred tocarry out the heat treatment at a low temperature at an initial stageand then increase the temperature stepwise or continuously.

The method of strictly controlling the post-drying step will be detailedhereinafter with reference to a generally employed method of forming afilm in the present invention. Generally, it is industrial practice toemploy a method in which a film is dried while the film is carried by apin tenter method or a roll suspension method. In this case, a filmexcellent in optical isotropy can be obtained by drying the film in astate where the film can be shrank in the width direction. The drying ispreferably carried out at a drying temperature T(°C.) in the range whichsatisfies the following inequality (I). As will be demonstrated inReferential Example 1 (FIG. 1), the glass transition temperature of anaromatic polyethersulfone greatly depends upon the residual solventamount, and remarkably decreases with an increase in the residualsolvent amount. When the drying temperature exceeds the upper limit, thefilm is liable to be undesirably deformed. In view of these points, inthis step, it is required to control the drying temperature strictly.

In the present invention, the drying is carried out at a dryingtemperature which satisfies the following inequality (I),

    Tg'-50<T<Tg'                                               (I)

wherein T (°C.) is a temperature in a dry atmosphere, and Tg'(°C.) is aglass transition temperature of an aromatic polyethersulfone containinga residual solvent, and this temperature increases with a decrease inthe residual solvent amount which decreases as the drying proceeds.

More preferably, the drying temperature satisfies Tg'-30<T<Tg'.

The above Tg' is obtained by measuring the sample sealed in a closedcell at a temperature elevation rate of 20° C./minute in accordance withDSC (differential scanning calorimetry) method. The Tg' is defined to bea temperature at which a second-order change in heat flow commences.

As described already, Tg' depends upon the residual solvent amount. Inthe post-drying step, as the film is carried on, the residual solventamount decreases, and Tg' accordingly increases. For efficiently, dryingthe film without causing a strain on the film, the temperature isincreased as Tg' increases. When the drying temperature is lower thanTg'-50° C., undesirably, the drying is not efficiently carried out. Whenit exceeds Tg', undesirably, a strain occurs.

In the present invention, air may be fed in the post-drying step as wellas in the pre-drying step.

In the above drying step, the drying means is selected from hot airdrying, electric heating and infrared heating. Further, microwaveheating is also used. The principle of the microwave heating is based onshaking dipoles contained in a substance with a microwave. Theabsorption efficiency of the microwave depends upon the magnitude of adipole moment and the easiness with which molecules are put in motion inagreement with the frequency of the microwave. Water is a typicalsubstance which satisfies both of them, and a microwave is put topractical use as a so-called electronic oven and also industrially usedfor drying a product containing water. However, the microwave is notnecessarily effective for general organic substances. The presentinventors focussed on the following. Since a 1,3-dioxolane molecule hastwo oxygen atoms as electronegative atoms and the molecule isstructurally asymmetric, the dipoles which the two oxygen atoms arerelated to do not compensate. In other words, the inventors noticed thata large dipole moment can be expected. On the other hand, the presentinventor also focussed on the fact that an aromatic polyethersulfonehas, in its molecule, a sulfone group having a large dipole moment. Amicrowave is accordingly applied to the drying of a film formed from adope of an aromatic polyethersulfone in 1,3-dioxolane. As a result, ithas been found that, surprisingly, the film can be dried veryefficiently and that a uniform film free of foaming and orange peelsurface can be obtained. The drying means used in the drying step in thepresent invention includes a microwave heating based on this discovery.

It is ideally preferred that the frequency of a microwave heatingapparatus used in the present invention is set at a frequency at which1,3-dioxolane and aromatic polyethersulfone molecules are both easilyput to motion. However, the frequency is generally restricted by RadioWave Law and a microwave electronic tube, and a general heatingapparatus uses a frequency of 2,450 MHz. However, 915 MHz can be used solong as it does not jam other communications. Under the circumstances,in the present invention, the frequency of 2,450 MHz and the frequencyof 915 MHz are preferably used. In the present invention, the microwaveheating may be used through the drying and heat treatment steps, or itmay be used in part of the steps. Further, the film may be dried by acontinuous method (conveyer oven method) or by a batch method. Themicrowave intensity is determined in consideration of the foaming,orange peel surface and undulation of the film.

In the present invention, the drying step, i.e., the removal of solventfrom the dope cast on the substrate, is generally carried out in airatmosphere, while it is preferred to carry out the drying step in aninert gas atmosphere.

The inert gas for constituting the above inert gas atmosphere includesnon-oxidizing incombustible gases such as nitrogen gas, argon gas,helium gas and carbon dioxide gas. Of these, nitrogen gas and carbondioxide gas are preferably used in views of economic performance.

The concentration of oxygen in the above inert gas is preferably 10% byvolume or less, more preferably 8% by volume or less, particularlypreferably 5% by volume or less. When the oxygen concentration is higherthan 10% by volume, undesirably, the possibility of an explosionincreases. The oxygen concentration can be set such that the aboveconditions are satisfied, and has no lower limit. The lower limit of theoxygen concentration may be determined as required in view of economicperformance.

In the above inert gas atmosphere, the concentration of vapor of thesolvent containing 1,3-dioxolane as a main component, contained in thedope, is preferably at least 3% by volume, more preferably at least 5%by volume in view of the efficiency of recovering the solvent and sincethe film is dried at a high rate. The upper limit of the aboveconcentration is not specially limited, while it is preferably 50% of asaturated vapor concentration at a drying temperature. When the aboveconcentration exceeds the above upper limit, undesirably, the dryingrate decreases.

The inert gas containing the solvent vapor having the aboveconcentration is introduced into a cooled condenser to recover thesolvent in an inert gas atmosphere, whereby oxidizable 1,3-dioxolane canbe stably recovered while preventing the formation of peroxide.

When the present invention is practiced, an inert gas source, such as anitrogen source, equipped with a flow adjusting device is connected toan air introducing portion of a dryer, and a cooling-condenser apparatusis connected to an air outlet, whereby the solvent can be recovered froma high concentration of atmosphere. An apparatus of this type is alreadytechnically disclosed (see JP-B-55-36389 and JP-B-59-21656).

Further, since most of the solvent is removed in the pre-drying step, itis not always necessary to carry out the post-drying step in the inertgas atmosphere, and it may be carried out in air. Even when thepost-drying step is carried out in the inert gas atmosphere, it isself-evident that the solvent concentration in the atmosphere ispreferably low. This step can be carried out according to thepost-drying step in an air-atmosphere mentioned before.

As described above, since the drying in the inert gas atmosphere is freefrom the problem of explosion limit, the drying can be carried out in anatmosphere containing a high concentration of the solvent, and thesolvent can be recovered by a simple condensation method. On the otherhand, in an air atmosphere, it is required to carry out the drying in anatmosphere having a low solvent concentration below the explosion limit,and an adsorption method or a gas absorption method is all that can beapplied. Therefore, the drying in an air atmosphere is unavoidablydisadvantageous in view of the recovery of the solvent and the formationof peroxide. The drying in an inert gas atmosphere is advantageous sincethe air oxidation of the solvent does not take place so that theformation of peroxide is prevented. Further, it has been generallybelieved that the drying proceeds more efficiently with a decrease inthe solvent vapor concentration. However, surprisingly, the dryingproceeds smoothly even in a high-concentration atmosphere in the presentinvention, and when the drying is evaluated as a whole including thepost-drying step, it has been found that the drying can be effected at arate which is not inferior to the drying rate in an atmosphere having alow solvent concentration in air.

Thus, according to the present invention, as described above, there isprovided an optically isotropic film of an aromatic polyethersulfone,which

(A) comprises an aromatic polyethersulfone,

(B) has a transmittance to visible light at a wavelength of 550 nm inthe range of from 86 to 91%,

(C) has a haze value in the range of from 0.1 to 1%,

(D) shows a refractive index difference (Δn) of 0.0010 or less, which isa difference between a refractive index (n_(s)) in the slow axisdirection in an in-plane area of the film and a refractive index (n_(f))in the fast axis direction in the in-plane area,

(E) has a refractive index ratio (n_(z) /n_(f)) of 0.9997 to 1.0000,which is a ratio (n_(z) /n_(f)) of a refractive index (n_(z)) in aperpendicular direction with regard to a film surface to the refractiveindex n_(f),

(F) has a thickness in the range of from 30 to 300 μm, and

(G) has a thickness nonuniformity in the range of from 0.1 to 1% basedon an average film thickness.

The optically isotropic film used in the present invention is requiredto have high transparency in use and optically high uniformity. Thetransmittance of the optically isotropic film to visible light atwavelength of 550 nm is in the range of 86 to 91%, preferably 87 to 90%.When the above transmittance is less than the above lower limit,undesirably, the light loss is large. When it exceeds the above upperlimit, it is required to decrease the film thickness for avoiding thedecrease of the transmittance due to absorption, and undesirably, themechanical strength adequate for practical use cannot be obtained. Thehaze value of the film is 0.1 to 1%, preferably 0.15 to 0.7%. When thehaze value exceeds the above upper limit, undesirably, the contrast ofan image decreases due to scattering. The haze caused by a scatteringwithin the film can be controlled by decreasing the film thickness.However, when the film thickness is decreased, undesirably, themechanical strength adequate for practical use cannot be obtained, asdescribed above.

When used as a substrate material for a liquid crystal display device,the optically isotropic film of the present invention performs a role ofaccurately transmitting polarized light which has passed through apolarization plate or polarized light which has passed through a liquidcrystal layer, and the film itself is therefore required to be opticallyuniform. The meaning of being optically uniform corresponds to a verysmall variability of the birefringence. That is, in the opticallyisotropic film of the present invention, the refractive index difference(Δn) between a refractive index (n_(s)) in the slow axis direction in anin-plane area of the film and a refractive index (n_(f)) in the fastaxis direction in the in-plane area is 0.0010 or less, preferably 0.0008or less. When Δn is greater than the above, polarized light has adistortion, and such film is not suitable as a substrate for a liquidcrystal display device.

The ratio (n_(z) /n_(f)) of the refractive index (n_(z)) in theperpendicular direction with regard to the film surface to therefractive index (n_(f)) in the fast axis direction is 0.9997 to 1.0000,preferably 0.9998 to 1.0000. When n_(z) /n_(f) is less than 0.9997,undesirably, the effect on the improvement of visual angle of visibilitygreatly decreases. The n_(z) /n_(f) ratio of greater than 1.0000 is hardor impossible to accomplish.

The average thickness of the optically isotropic film of the presentinvention is in the range of from 30 to 300 μm, preferably 50 to 200 μm.For a liquid crystal display device, preferred is the film having anaverage thickness of 50 to 200 μm. When the average thickness is largerthan the above upper limit, it is difficult to remove the residualsolvent. When it is smaller than the above lower limit, it is difficultto prevent the thickness nonuniformity. Further, the thicknessnonuniformity greatly influences the optical properties. The thicknessnonuniformity in this case corresponds to a fine uneven surfaceattributed to the roughness on the surface. In the presentspecification, the term "thickness nonuniformity" is defined to be adifference between the highest peak and the deepest bottom in thethickness of a film when the film is traced 1 cm in any place on thefilm. The measurement method therefor is not specially limited, while aneedle-contact method is generally employed. The thickness nonuniformityof the optically isotropic film of the present invention is 0.1 to 1%,frequently 0.2 to 0.8%, of the average film thickness.

According to the present invention, the above optically isotropic filmmay be uniaxially stretched.

The uniaxial stretching may be carried out by any one of a method ofuniaxial stretching in a longitudinal direction, a method of uniaxialstretching with a tenter in a transverse direction and a roll stretchingmethod. The stretching temperature depends upon the Tg of the film used,and it is generally between (Tg-50° C.) and (Tg+30° C.), preferablybetween (Tg-30° C.) and (Tg+20° C.). When the stretching temperatureexceeds the above upper limit, undesirably, the polymer undergoesorientation relaxation and the stretching effect is greatly reduced.When it is lower than the above lower limit, undesirably, it isdifficult to accomplish uniform orientation since the molecular movementof the polymer is frozen. The stretch ratio is properly selecteddepending upon the magnitude of retardation of the film. Generally, thestretching is carried out such that the length of the stretched film is1.05 to 2.0 times, preferably 1.1 to 1.5 times, as large as theunstretched film. When the stretch ratio exceeds the above upper limit,undesirably, the phase difference (retardation) Re=Δn·d(Δn=birefringence, d=film thickness) is large to excess. When it is lessthan the above lower limit, undesirably, the phase difference is smallto excess.

According to the present invention, there is further provided anoptically anisotropic, uniaxially, oriented film of an aromaticpolyethersulfone, which

(A) comprises an aromatic polyethersulfone,

(B) has a transmittance to visible light at a wavelength of 550 nm inthe range of from 86 to 91%,

(C) has a haze value in the range of from 0.1 to 1%,

(D) shows a refractive index difference (Δn) in the range of from 0.0013to 0.0230, which is a difference between a refractive index (n_(s)) inthe slow axis direction in an in-plane area of the film and a refractiveindex (n_(f)) in the fast axis direction in the in-plane area,

(F) has a thickness in the range of from 30 to 300 μm, and

(G) has a thickness nonuniformity in the range of from 0.1 to 1% basedon an average film thickness.

The optically anisotropic uniaxially oriented film of the presentinvention is suitably used as a phase difference film for a liquidcrystal display device. In this sense, the transmittance and the hazevalue of the optically anisotropic uniaxially oriented film are the sameas those of the already described optically isotropic film of thepresent invention. That is, the transmittance is 86 to 91%, preferably87 to 90%. The haze value is 0.1 to 1%, preferably 0.15 to 0.7%. On theother hand, the refractive index difference (Δn) between a refractiveindex (n_(s)) in the slow axis direction in an in-plane area of the filmand a refractive index (n_(f)) in the fast axis direction in thein-plane area is 0.0013 to 0.0230, preferably 0.0015 to 0.0200. When therefractive index difference is greater than the above upper limit orsmaller than the above lower limit, undesirably, no intended phasedifference effect can be obtained.

The average thickness of the optically anisotropic uniaxially, orientedfilm of the present invention is in the range of from 30 to 300 μm,preferably 50 to 200 μm. When the average thickness is larger than theabove upper limit, it is difficult to prevent the thicknessnonuniformity. The thickness nonuniformity of the uniaxially orientedfilm is 0.1 to 1%, frequently 0.2 to 0.8%, based on the average filmthickness.

The optically anisotropic uniaxially oriented film of the presentinvention is suitably used as a phase difference film for a liquidcrystal display device.

The above film is required to have a large birefringence, while thevariability of the birefringence is required to be small. For thispurpose, it is required that the optical uniformity of the film isextremely high at the stage of the unstretched film. In view of thesepoints, the above optically isotropic film having Δn of 0.0010 or less,preferably 0.0008 or less, is advantageously used.

Further, the phase difference (Re) at a wavelength of 590 nm, used inthe present invention, is in the range of from 400 to 700 nm, preferably420 to 650 nm, and the phase difference variability (ΔRe) is 0.01 to 2%,preferably 0.02 to 1.5%, based on the phase difference. When the phasedifference variability is outside the above range, undesirably, thephase difference effect of a liquid crystal display device cannot beobtained. Further, when the phase difference variability exceeds theabove upper limit, undesirably, a uniform image, especially in terms ofcolor and contrast, can not be obtained. When it is less than the abovelower limit, undesirably, it is difficult to control the phasedifference variability. Here, ΔRe is a variability of Re measured 5times at various portions in an area of 10 cm×10 cm in square of thefilm.

According to the present invention, there can be obtained an aromaticpolyethersulfone film which is excellent in surface properties, opticalproperties and uniformity and has a reduced residual solvent by acasting method, in the presence of 1,3-dioxolane as a main solvent,which is a halogen-free solvent free from causing environmentalpollution and corrosion. The obtained film is useful as an optical film,particularly as a phase difference film, for a liquid crystal displaydevice.

The present invention will be detailed hereinafter with reference toExamples. Measurements in Examples were carried out as follows.

Solution viscosity: Measured at 30° C. with a Brookfield viscometer BHmodel supplied by Tokyo Keiki K.K.

Glass transition temperature: TA Instruments DSC 2920 DifferentialScanning Calorimeter.

Heat shrinkage percentage: A film having a length of 20 cm washeat-treated at a predetermined temperature for a predetermined periodof time, and a dimensional change between the film before the heattreatment and the film after the heat treatment was determined.

Film thickness: Measured with a needle-contact type film-thicknessmeasuring meter supplied by Anritsu K.K.

Transmittance: Measured with an ultraviolet visible light spectrometer(UV-240) supplied by Shimadzu Corporation.

Haze value: Measured with an automatic digital haze meter UDH-20Dsupplied by Nippon Denshoku Kogyo K.K.

Phase difference: Measured with an automatic birefringence meterKOBURA-21ADH supplied by KS Systems K.K.

Quantitative determination of residual solvent: A sample film was heatedat 200° C. in a nitrogen atmosphere overnight. The sample film wasmeasured for weights before the heating and after the heating.

Quantitative determination of peroxide: Determined by a titrationmethod.

EXAMPLE 1

10 Parts by weight of an aromatic polyethersulfone η_(sp) /c=0.33 dl/g(0.5 g/l in 1,3-dioxolane at 30° C.)! containing 78% of recurring unitof the formula (1) in which each of Ar¹ and Ar² was a p-phenylene groupand 22% of recurring unit of the formula (3) in which each of Ar⁷, Ar⁸and Ar⁹ was a p-phenylene group was gradually added to, and dissolvedin, 23.3 parts by weight of 1,3-dioxolane with stirring at 50° C., togive a transparent and viscous dope. The dope had a solution viscosityof 2.8×10³ cps at 30° C. This solution showed no change when allowed tostand in a closed state at room temperature for 1 week.

The above dope was filtered through a filter having an opening diameterof 5 μm, and cast on a fully cleaned ferrotype substrate with a doctorblade. Then, the cast dope was heated at 65° C. for 15 minutes and at90° C. for 10 minutes to dry it, and the resultant film was peeled offfrom the substrate. The peeled film was heat-treated at 120° C. for 20minutes, at 150° C. for 30 minutes and at 200° C. for 120 minutes togive a transparent film having a thickness of 98 μm. The so-obtainedfilm was free of foaming, an orange peel surface and a wavingphenomenon, and was uniform. Further, the residual solvent amount was0.3% by weight. The film had a thickness nonuniformity of 0.43 am andwas very uniform. The transmittance of the film to visible light at awavelength of 550 nm was 88%, and the film had a haze value of 0.5% andhad very high transparency. The average refractive index of the film,measured with sodium D ray, was 1.6500, and the difference between therefractive index (n_(s)) of the film in the slow axis direction and therefractive index (n_(f)) of the film in the fast axis direction was lessthan 0.0001. Thus, the film was highly optically isotropic. The film wasmeasured for a phase difference at a wavelength of 590 nm to show lessthan 10 nm, and the variability in the film was small. Further, thevariability in the slow axis was ±10° or smaller, and the film wasoptically uniform. The film was measured for a glass transition point byDSC to show 213° C. and thus had very high heat resistance. Further, thefilm showed an elastic modulus of 238 kg/mm², a break strength of 8.6kg/mm² and an elongation at breakage of 15%, and thus had sufficientstrength. The film was measured for heat shrinkage at 100° C./30 minutesand at 150° C./30 minutes to show 0.03% in both the cases, and thus hadhigh dimensional stability.

The above-obtained film was uniaxially stretched 1.1 times at 195° C.The stretched film had a thickness of 95 μm and a thicknessnonuniformity of 0.48 μm. Further, the film showed a phase difference(Re) 520 nm and a phase difference variability of 8 nm. The Δn value ofthe film was 0.0055 nm. The ratio of the phase difference (Re) at awavelength of 450 nm to the phase difference at a wavelength of 550 nmwas 1.14, and the dependency of the phase difference on wavelength washigh as compared with polycarbonate (1.08), and the film was found to beeffective as a phase difference film for fast-responding liquid crystaldisplay.

Comparative Example 1

An attempt was made to prepare a dope in the same manner as in Example 1except that 1,3-dioxolane was replaced with 1,4-dioxane ortetrahydrofuran, but no homogeneous solution was obtained.

EXAMPLE 2

A dope was prepared in the same manner as in Example 1 except that 23.3parts by weight of 1,3-dioxolane was replaced with 30 parts by weight of1,3-dioxolane. The dope had a solution viscosity of 1.4×10³ cps at 30°C.

A film having a thickness of 86 μm was obtained from the above dope by adoctor blade method in the same manner as in Example 1. The film wasfree of foaming, an orange peel surface and a waving phenomenon and wasuniform. The residual solvent amount thereof was very small, as small as0.3%. The transmittance of the film to visible light was 85%, and thefilm had a haze value of 0.6% and had very high transparency. The filmwas measured for a phase difference at a wavelength of 590 nm to showless than 10 nm, and the variability in the film was small. Further, thevariability in the slow axis was ±10° or smaller, and the film wasoptically uniform. The film was measured for a glass transition point byDSC to show 210° C. and thus had very high heat resistance.

Comparative Example 2

A dope was prepared in the same manner as in Example 1 except that 23.3parts by weight of 1,3-dioxolane was replaced with 57 parts by weight of1,3-dioxolane. The dope was stable. The dope had a solution viscosity ofless than 100 cps and thus had a very low viscosity. A film was formedfrom the dope by a doctor blade method in the same manner as in Example1 to give a very poor film having a thickness of about 22 μm. The filmhad an orange peel-like unevenness all over the surface and was notsuitable for use.

Comparative Example 3

An attempt was made to prepare a dope in the same manner as in Example 1except that 23.3 parts by weight of 1,3-dioxolane was replaced with 13parts by weight of 1,3-dioxolane. However, the dope contained a largeresidual insoluble content. When the dope containing the residualinsoluble content was allowed to stand at room temperature for 24 hours,the dope formed a gel as a whole to lose its flowability. An attempt wasmade to form a film from the dope in the same manner as in Example 1immediately after the dope was prepared, but no homogeneous film wasobtained.

EXAMPLE 3

Parts by weight of an aromatic polyethersulfone η_(sp) /c=0.5 dl/g (0.5g/l in 1,3-dioxolane at 30° C.)! formed of recurring unit of the formula(2) in which each of Ar³ to Ar⁶ was a p-phenylene group and X was a2,2-propylene group --C(CH₃)₂ --! was dissolved in 30 parts by weight of1,3-dioxolane with stirring at 50° C., to give a transparent and viscousdope. The dope had a solution viscosity of 3.2×10³ cps at 30° C. Thissolution was aged at room temperature for 4 days to precipitate a whitecrystal of a cyclic oligomer. This crystal was removed by filtrationwith a filter having an opening diameter of 5 μm, and the dope was caston a fully cleaned ferrotype substrate with a doctor blade. Then, thecast dope was heated at 65° C. for 10 minutes and at 90° C. for 10minutes to dry it, and the resultant film was peeled off from thesubstrate. The peeled film was heat-treated at 120° C. for 20 minutes,at 150° C. for 10 minutes and at 180° C. for 120 minutes to give atransparent film having a thickness of 103 μm. The so-obtained film wasfree of foaming, an orange peel surface and a waving phenomenon and wasuniform. Further, the residual solvent amount was 0.3% by weight. Thefilm had a thickness nonuniformity of 0.39 μm and was very uniform. Thetransmittance of the film to visible light at a wavelength of 550 am was89.1%, and the film had a haze value of 0.3% and had very hightransparency. The average refractive index of the film, measured withsodium D ray, was 1.6334, and the difference between the refractiveindex (n_(s)) of the film in the slow axis direction and the refractiveindex (n_(f)) of the film in the fast axis direction was less than0.0001. Thus, the film was highly optically isotropic. The ratio (n_(z)/n_(f)) of the refractive index (n_(z)) in the perpendicular directionwith regard to the film surface to the refractive index (n_(f)) in thefast axis direction was 0.9999, and the film thus had highthree-dimensional optical isotropy including the isotropy in theperpendicular direction. The film was measured for a phase difference ata wavelength of 590 nm to show less than 10 nm, and the variability inthe film was small. Further, the variability in the slow axis was ±10°or smaller, and the film was optically uniform. The film was measuredfor a glass transition point by DSC to show 189° C. and thus had veryhigh heat resistance. Further, the film showed an elastic modulus of 225kg/mm², a break strength of 7.0 kg/m² and a elongation at breakage of37%, and thus had sufficient strength. The film was measured for heatshrinkage percentages at 100° C./30 minutes and at 150° C./30 minutes toshow 0.02% in both the cases, and thus had high dimensional stability.

The above-obtained film was uniaxially stretched 1.1 times at 191° C.The stretched film had a thickness of 100 μm and a thicknessnonuniformity of 0.45 μm. Further, the film showed a phase difference(Re) 580 nm and a phase difference variability of 8 nm. The Δn value ofthe film was 0.0058 nm. The ratio of the phase difference (Re) at awavelength of 450 nm to the phase difference at a wavelength of 550 nmwas 1.14, and the dependency of the phase difference on wavelength washigh as compared with polycarbonate (1.08). The film was thus found tobe effective as a phase difference film for fast-responding liquidcrystal display.

EXAMPLE 4

10 Parts by weight of an aromatic polyethersulfone η_(sp) /c=0.41 dl/g(0.5 g/l in 1,3-dioxolane at 30° C.)! formed of recurring unit of theformula (2) in which each of Ar³ to Ar⁶ was a p-phenylene group and Xwas a 2,2-propylene group --C(CH₃)₂ --! was dissolved in 27 parts byweight of 1,3-dioxolane with stirring at 50° C., to give a transparentand viscous dope. The dope had a solution viscosity of 2.3×10³ cps at30° C. This solution was aged at room temperature for 4 days toprecipitate a white crystal of a cyclic oligomer. This crystal wasremoved by filtration with a filter having an opening diameter of 5 μm .A film having a thickness of 93 μm was formed from the dope by a doctorblade method in the same manner as in Example 3. The film was free offoaming, an orange peel surface and a waving phenomenon, and wasuniform. The residual solvent amount thereof was very small, as small as0.3%. The transmittance of the film to visible light was 89.3%, and thefilm had a haze value of 0.4% and was optically transparent. The filmwas measured for a phase difference at a wavelength of 590 nm to showless than 10 nm, and the variability in the film was small. Further, thevariability in the slow axis was ±10° or smaller, and the film wasoptically uniform. The film was measured for a glass transition point byDSC to show 189° C. and thus had very high heat resistance. The film wasmeasured for heat shrinkage at 100° C./30 minutes and at 150° C./30minutes to show 0.02% in both the cases, and thus had high dimensionalstability.

EXAMPLE 5

A dope was prepared in the same manner as in Example 4 except that 27parts by weight of 1,3-dioxolane was replaced with 40 parts by weight of1,3-dioxolane. The dope had a solution viscosity of 8.0×10² cps. Thissolution was filtered and formed into a film in the same manner as inExample 3 to give a film having a thickness of 53 μm. The film was freeof foaming, an orange peel surface and a waving phenomenon. The film hada thickness nonuniformity of 0.38 μm, and was uniform. The transmittanceof the film to visible light was 89.4%, and the film had a haze value of0.2% and was thus optically transparent. The film was measured for aphase difference at a wavelength of 590 nm to show less than 10 nm, andthe variability in the film was small.

Comparative Example 4

A dope was prepared in the same manner as in Example 5 except that 40parts of 1,3-dioxolane was replaced with 40 parts of tetrahydrofuran.The dope had a very low solution viscosity, as low as 3.2×10² cps. Thedope was filtered, and then formed into a film. The film had an orangepeel surface all over and was not suitable for use. Further, since thedope had a low viscosity, the film thickness even in its thicker portionwas as small as 35 μm, and the film had a thickness nonuniformity of asmuch as 3.5 μm.

EXAMPLE 6

10 Parts by weight of an aromatic polyethersulfone η_(sp) /c=0.30 dl/g(0.5 g/l in 1,3-dioxolane at 30° C.)! containing 78% by weight ofrecurring unit of the formula (1) in which each of Ar¹ and Ar² was ap-phenylene group and 22% by weight of recurring unit of the formula (3)in which each of Ar⁷, Ar⁸ and Ar⁹ was a p-phenylene group was dissolvedin 35 parts by weight of 1,3-dioxolane containing 10% by weight of1,4-dioxane, tetrahydrofuran or cyclohexanone with stirring at 50° C.,to give transparent and viscous dopes. Films having a thickness of about60 μm were formed from the dopes in the same manner as in Example 1.These films were free of foaming, an orange peel surface and a wavingphenomenon, and were optically uniform. The transmittance of each of thefilms obtained from the dope containing 1,4-dioxane, tetrahydrofuran orcyclohexanone in combination was 87.9, 88.0 or 88.0%, and each film hada haze value of 0.4% and was remarkable highly transparent.

EXAMPLES 7-10

The aromatic polyethersulfones used in Examples 1 and 3 were tested fortheir peelings from a substrate. A fully cleaned and dried, freshferrotype substrate was used as the substrate. The polymer used inExample 1 was dissolved in 1,3-dioxolane which further contained apredetermined solvent. The resultant dope was filtered in the samemanner as in Example 1, and then cast on the substrate. The cast dopewas dried at 65° C. for 15 minutes and at 90° C. for 10 minutes to forma film, and then the film was peeled off from the substrate. The aboveoperation procedure was repeated using the same substrate to determinehow many times the peeling was possible, and the upper limit of thenumber of times of the peeling that was possible was taken as a numberof times of peeling. The polymer used in Example 3 was dried under heatat 65° C. for 10 minutes and at 90° C. for 10 minutes, and then peeledoff from the same substrate as the above. Table 1 shows the results. Asis clear in Table 1, when a proper amount of water, ethanol orisopropanol was added to 1,3-dioxolane, the films were excellent in theproperty of being peeled off from the substrate.

                  TABLE 1                                                         ______________________________________                                                      Solvent    Number of times                                      Polymer       (%)        of peeling                                           ______________________________________                                        Ex. 7   PSF       water (3)  5<                                               Ex. 8   PSF       ethanol (3)                                                                              5<                                               Ex. 9   PSF       isopropanol (3)                                                                          5<                                               Ex. 10  PES       water (3)  5<                                               ______________________________________                                         Ex. = Example                                                                 PES = Aromatic polyethersulfone used in Example 1                             PSF = Aromatic polyethersulfone used in Example 3                        

EXAMPLE 11

A dope (A) in 1,3-dioxolane which was the same as that in Example 3 wasprepared in the same manner as in Example 3, and a dope (B) in1,3-dioxolane further containing 3% of water, which was the same as thatin Example 10 was prepared in the same manner as in Example 10.Immediately after the dopes were prepared, the dopes were placed in 1 cmcells and measured for a haze value to show 0.1% each. When the dopeswere allowed to stand at room temperature for 4 days, white precipitateswere formed. The dopes were filtered through a filter having an openingdiameter of 0.5 μm to show a haze value of 0.1% each. Further, when thefiltrates were allowed to stand at room temperature for 2 days, thefiltrate from the dope (A) had a haze value of 34%, while the filtratefrom the dope (B) had a haze value of 0.1% and showed no change.

Referential Example 1

Films having a thickness of about 100 μm and having different residualsolvent amounts were formed from the same dopes as those in Examples 1and 3 by changing the drying conditions. FIG. 1 shows the glasstransition points (Tg') of these films. In FIG. 1, curves A and Bcorrespond to curves from the dopes in Examples 1 and 3. As is clear inFIG. 1, Tg' remarkably decreased with an increase in the residualsolvent amounts.

EXAMPLE 12

A film was continuously formed from the same dope as that in Example 3.A casting apparatus had a system in which a film was extruded through adie onto a belt and the belt was connected to a drying furnace which wasseparated to 4 zones. Further, a heat-treatment furnace (post-dryingfurnace) had a system in which the film peeled off from the belt wastreated in a furnace which was separated to 6 zones. The dope was castwith this apparatus, and then the temperature in the pre-drying furnacewas stepwise elevated to 40° C. (no air current), 65° C. (air current 1m/second) and 90° C. (air current 5 m/second), followed by final coolingat 40° C. The film was prepared as a self-supporting film having aresidual solvent amount of 10% by weight. At this stage, the film waspeeled off from the belt, and transferred to the post-drying furnace. Inthe post-drying furnace, in a state where the film was shrinkable in thewidth direction, the temperature was stepwise elevated, according toresidual solvent amounts, to 75° C. (residual solvent amount 10% byweight, Tg'=85° C.), 115° C. (residual solvent amount 5% by weight,Tg'=120° C.), 150° C. (residual solvent amount 2% by weight, Tg'=160°C.), 165° C. (residual solvent amount 1.5% by weight, Tg'=170° C.), 170°C. (residual solvent amount 1% by weight, Tg'=180° C.) and 180° C.(residual solvent amount 0.5% by weight, Tg'=185° C.) to obtain a dryfilm. The so-obtained film had a residual solvent amount of 0.3% byweight. The film had a thickness of 100 μm and a thickness nonuniformityof 0.25 μm and was remarkably uniform. The transmittance of the film tovisible light at a wavelength of 550 nm was 89.2%, and the film had ahaze value of 0.2% and was highly transparent. The average refractiveindex of the film, measured with sodium D ray, was 1.6334, and thedifference between the refractive index (n_(s)) of the film in the slowaxis direction and the refractive index (n_(f)) of the film in the fastaxis direction was less than 0.0001. Thus, the film was highly opticallyisotropic. The film was measured for a phase difference at a wavelengthof 590 nm to show less than 10 nm, and the variability in the film wassmall.

EXAMPLE 13

A film was continuously formed from the same dope as that in Example 1.The same casting apparatus as that used in Example 12 was used. The dopewas cast with this apparatus, and then the temperature in the pre-dryingfurnace was stepwise elevated to 40° C. (no air current), 65° C. (aircurrent 2 m/second) and 90° C. (air current 5 m/second) , followed byfinal cooling at 40° C. The film was prepared as a self-supporting filmhaving a residual solvent amount of 12% by weight. At this stage, thefilm was peeled off from the belt, and transferred to the post-dryingfurnace. In the post-drying furnace, the temperature was stepwiseelevated, according to residual solvent amounts, to 85° C. (residualsolvent amount 12% by weight, Tg'=90° C.), 110° C. (residual solventamount 7% by weight, Tg'=120° C.) 155° C. (residual solvent amount 3% byweight, Tg'=160° C.), 175° C. (residual solvent amount 1.5% by weight,Tg'=180° C.), 190° C. (residual solvent amount 1% by weight, Tg'=200°C.) and 200° C. (residual solvent amount 0.5% by weight, Tg'=210° C.) toobtain a dry film. The so-obtained film had a residual solvent amount of0.3% by weight. The film had a thickness of 103 μm and a thicknessnonuniformity of 0.42 μm and was remarkably uniform. The transmittanceof the film to visible light at a wavelength of 550 nm was 88.2%, andthe film had a haze value of 0.3% and was highly transparent. Theaverage refractive index of the film, measured with sodium D ray, was1.6500, and the difference between the refractive index (n_(s)) of thefilm in the slow axis direction and the refractive index (n_(f)) of thefilm in the fast axis direction was less than 0.0001. Thus, the film washighly optically isotropic. The ratio (n_(z) /n_(f)) of the refractiveindex (n_(z)) in the perpendicular direction with regard to the filmsurface to the refractive index (n_(f)) in the fast axis direction was0.9999, and the film thus had high three-dimensional optical isotropyincluding the isotropy in the perpendicular direction. The film wasmeasured for a phase difference at a wavelength of 590 nm to show lessthan 10 nm, and the variability in the film was small.

EXAMPLE 14

The same solution of an aromatic polyethersulfone in 1,3-dioxolane asthat prepared in Example 1 was cast on a fully cleaned ferrotypesubstrate by a doctor blade method. Then, the cast solution was heatedin a nitrogen gas atmosphere containing 12% by volume of 1,3-dioxolane,at 65° C. for 15 minutes and then heated at 90° C. for 10 minutes. Theformed film was peeled off from the substrate. Then, in air, the peeledfilm was heat-treated at 120° C. for 20 minutes, at 150° C. for 30minutes and at 200° C. for 120 minutes to give a transparent film havinga thickness of 99 μm. The so-obtained film was free of foaming, anorange peel surface and waving and was uniform. Further, the residualsolvent amount was 0.2% by weight, which was almost equivalent to thefilm obtained in Example 1 under the same conditions in an airatmosphere having a low solvent concentration. The transmittance of thefilm to visible light at a wavelength of 550 nm was 88.1%, and the filmhad a haze value of 0.4% and had very high transparency.

Further, 1,3-dioxolane in a drying furnace having a nitrogen gasatmosphere was recovered through an exhaust outlet, trapped at -70° C.and measured for an amount of peroxide contained therein. The result wasthat the recovered 1,3-dioxolane had a peroxide amount of 109 ppm, whilethe 1,3-dioxolane used had a peroxide amount of 100 ppm. It wastherefore shown that almost no peroxide was formed during the filmformation.

EXAMPLE 15

The same dope as that prepared in Example 3 was cast on a glasssubstrate, and dried in a 2,450 MHz microwave heating apparatus at 100 Wfor 3 minutes, at 200 W for 7 minutes, at 300 W for 3 minutes and at 500W for 10 minutes. The resultant film was peeled, and then heat-treatedat 120° C. for 20 minutes, at 150° C. for 10 minutes and further at 180°C. for 60 minutes. The film had a residual solvent amount of 0.5%. Thetransmittance of the film to visible light at a wavelength of 550 nm was89.2%, and the film had a haze value of 0.5% and was highly transparent.The film was measured for a phase difference at a wavelength of 590 nmto show less than 10 nm, and the variability in the film was small.Further, the variability in the slow axis was ±10° or smaller, and thefilm was optically uniform. The film was measured for a glass transitionpoint by DSC to show, 188° C. and thus had very high heat resistance.The film showed an elastic modulus of 218 kg/mm², a break strength of7.1 kg/mm² and an elongation at breakage of 35%, and thus had sufficientstrength. The film was measured for heat shrinkage percentages at 100°C./30 minutes and at 150° C./30 minutes to show 0.04% in both the cases,and thus had high dimensional stability.

EXAMPLE 16

A dope was prepared by dissolving 10 parts by weight of an aromaticpolyethersulfone in 24.5 parts by weight of 1,3-dioxolane containing 3%by weight of methanol in the same manner as in Example 5. The dope had asolution viscosity of 3.3×10³ cps. The dope was cast on a glasssubstrate, and dried under heat at 65° C. for 15 minutes and at 90° C.for 10 minutes, and the resultant film was peeled off from thesubstrate. The film was dried in a 2,450 MHz microwave heating apparatusat 500 W for 60 minutes. The film was free of foaming, an orange peelsurface and waving, and was very uniform. The film had a residualsolvent amount of 6%. The peeled film was further heat-treated at 180°C. for 120 minutes. The resultant film had a residual solvent amount of0.4% and was thus highly transparent. The film was measured for a phasedifference at a wavelength of 590 nm to show less than 10 nm, and thevariability in the film was small. Further, the variability in the slowaxis was ±10° or smaller, and the film was optically uniform. The filmwas measured for a glass transition point by DSC to show 192° C. andthus had very high heat resistance.

EXAMPLE 17

Parts by weight of an aromatic polyethersulfone η_(sp) /c=0.42 dl/g (0.5g/l in 1,3-dioxolane at 30° C.)! containing 24% of recurring unit of theformula (1) in which each of Ar¹ and Ar² was a p-phenylene group and 76%of recurring unit of the formula (2) in which each of Ar³ to Ar⁶ was ap-phenylene group and X was a 2,2-propylene group --C(CH₃)₂ --! wasdissolved in 30 parts by weight of 1,3-dioxolane with stirring at 50°C., to give a transparent and viscous dope. The dope had a solutionviscosity of 1.3×10³ cps at 30° C. The dope was filtered through afilter having an opening diameter of 5 μm, and cast by a doctor blademethod in the same manner as in Example 1, and the cast dope was driedat 65° C. for 30 minutes and at 90° C. for 10 minutes, peeled off fromthe substrate and then dried at 120° C. for 15 minutes, at 150° C. for30 minutes and at 200° C. for 60 minutes to give a transparent filmhaving a thickness of 93 μm. The film was free of foaming, an orangepeel surface and a waving phenomenon and was uniform. The residualsolvent amount thereof was very small, as small as 0.4%. The film had athickness nonuniformity of 0.38 μm and was very uniform. Thetransmittance of the film to visible light was 87.3%, and the film had ahaze value of 0.6% and was optically transparent. The film was measuredfor a phase difference at a wavelength of 590 nm to show less than 10nm, and the variability in the film seas small. Further, the film wasmeasured for a glass transition point by DSC to show 193° C. and thushad very high heat resistance. The film was measured for heat shrinkagepercentages at 100° C./30 minutes and at 150° C./30 minutes to shore0.02% in both the cases, and thus had high dimensional stability.

What is claimed is:
 1. An aromatic polyethersulfone solution compositioncomprising 15 to 40 parts by weight of a solvent which contains at least60% by weight of 1,3-dioxolane and can dissolve an aromaticpolyethersulfone and 10 parts by weight of the aromaticpolyethersulfone.
 2. The solution composition of claim 1, wherein thesolvent contains at least 60% by weight of 1,3-dioxolane and 40% byweight or less of an organic solvent compatible with 1,3-dioxolane. 3.The solution composition of claim 1, wherein the solvent furthercontains 1 to 10% by weight, based on the solvent, of at least one ofwater and a linear or branched aliphatic alcohol having 1 to 6 carbonatoms.
 4. The solution composition of claim 1, wherein the solutioncomposition is a dope used for producing a film by a solution castingmethod.
 5. The solution composition of claim 1 comprising 20 to 40 partsby weight of a solvent which contains at least 70% by weight of1,3-dioxolane and can dissolve the aromatic polyethersulfone and 10parts by weight of the aromatic polyethersulfone.
 6. The solutioncomposition of claim 5, wherein the solvent further contains 2 to 8% byweight, based on the solvent, of at least one of water and a linear orbranched aliphatic alcohol having 1 to 6 carbon atoms.
 7. An aromaticpolyethersulfone solution casting composition comprising a sufficient 15to 40 parts by weight of a solvent consisting essentially of1,3-dioxolane to dissolve the aromatic polyethersulfone and 10 parts byweight of the dissolved aromatic polyethersulfone.
 8. The solutioncomposition of claim 7, wherein the solvent further contains 2 to 8% byweight, based on the solvent, of at least one of water and an aliphaticalcohol having 1 to 4 carbon atoms.
 9. The solution composition of claim7 comprising 20 to 35 parts by weight of the 1,3-dioxolane solvent. 10.The solution composition of claim 9, wherein the solvent furthercontains 2 to 8% by weight, based on the solvent, of at least one ofwater and an aliphatic alcohol having 1 to 4 carbon atoms.